Dropbox Syncing

My desktop (primary), laptop and iPhone were syncing successfully. I then re-installed Windows on the laptop and installed MD from a fresh download.
I am following the advise in http://help.infinitekind.com/kb/syncing-and-sharing-data/syncing-with-other-computersLaptop MD shutdown. Primary syncing stopped. Dropbox shared folder deleted.
However, re-opening MD on the desktop (primary) and choosing File|Syncing, I am not offered the Dropbox Folder option depicted in the help. (the other options are present).
Where to go from here ?

dwg: Thanks I was a half step ahead of you. Desktop and Laptop are now syncing via Dropbox shared folder. The next step is to re-establish syncing to the iPhone. It wants a 'Network Synchronizer' extension that does not appear in the extensions manager. I could perhaps add it from file if I knew where to find the file !